<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
</head>
<body>
    <script>
        /* 用户输入一个数字n，计算1+2+3+4+……n的和 */
        var n = prompt("请输入一个数字");
        var add = 0;
        for(var i = 1 ; i <= n ; i++){
            add += i;
        }
        console.log(add);
    </script>
</body>
</html>